Question: $ { {3} \times \left[\begin{array}{r}{1} \\ {-1} \\ {3}\end{array}\right] = {?} } $
To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{r}{3}\times{1} \\ {3}\times{-1} \\ {3}\times{3}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{r}{3} \\ {-3} \\ {9}\end{array}\right]}$